A. Technical Field
The present invention relates to a water-absorbent resin powder and its production process and uses.
B. Background Art
Water-absorbent resins are used as absorbing agents for sanitary materials, such as sanitary napkins and disposable diapers. In addition, they have wide uses (e.g. they are used as such as water-retaining agents and dehydrating agents in agricultural and horticultural fields and engineering works fields).
Although not especially limited to the above-exemplified uses, it is needless to say that the water absorption performance of the water-absorbent resins must not be deteriorated with the passage of time.
As to devices to inhibit the deterioration of the water absorption performance, known examples of methods for inhibiting the water-absorbent resins from being deteriorated due to urine with the passage of time include counter-methods as described in such as JP-A-118375/1988, U.S. Pat. Nos. 4,972,019, 4,863,989, JP-A-127754/1988, JP-A-153060/1988, JP-A-272349/1988, JP-A-033158/1989, JP-A-255804/1990, JP-A-179008/1991, EP 0372981, and JP-A-097929/1993.
However, on the present inventors"" investigations, it has been found that the deterioration of the water absorption performance is caused even if these conventional counter-methods are carried out.
A. Object of the Invention
Accordingly, an object of the present invention is: to search for the hitherto unknown cause of the above deterioration of the water absorption performance; and then to try its prevention.
B. Disclosure of the Invention
The present inventors diligently studied to solve the above-mentioned problems. As a result, they have found out that a water-absorbent resin includes a trace of metallic foreign substance. Particularly, they have found that: when the water-absorbent resin is divided into small particles in order to enhance the performance of the water-absorbent resin and besides, the fine-powder content is lowered by narrowing the particle diameter distribution, then the content of the above metallic foreign substance increases, with the result that the metallic foreign substance as contained in a trace promotes the deterioration of the water absorption performance of the water-absorbent resin. Specifically, the present inventors have found out that: the above deterioration is particularly easily caused when an attempt is made for the water-absorbent resin powder to have a mass-average particle diameter of 300 to 600 xcexcm and for the ratio of fine particles having particle diameters of not larger than 150 xcexcm to decrease to less than 10 mass % of the water-absorbent resin powder so that the performance of the water-absorbent resin can be enhanced; and the more the amount of the metallic foreign substance as removed from the water-absorbent resin powder is increased, the more effectively the above deterioration can be prevented.
The present inventors have found out that: in a step of transferring a dried product powder of a crosslinked hydrogel polymer that will be a water-absorbent rein powder, the removal of the metallic foreign substance from the above water-absorbent rein powder can be easily and certainly carried out by passing a specific magnetic field through this dried product powder, with the result that the properties of the water-absorbent rein powder can be improved greatly.
In addition, the present inventors have found out that the cause of generating the metallic foreign substance is the following.
When a water-absorbent resin powder having a narrow particle diameter distribution and a low fine-powder content is industrially mass-produced, a dried product of a crosslinked hydrogel polymer is usually continuously pulverized, and then continuously transferred to a classification step and continuously classified. The metallic foreign substance is generated as a result that the dried product powder hurts metallic faces when contacting them on such a production line.
The metallic foreign substance is easily generated when not less than 50% of faces, contacting the water-absorbent resin powder, of apparatuses as used for the above steps or other steps on the production line are stainless steel faces.
The water-absorbent resin powder is favorably produced by a process including the steps of: drying a hydrogel polymer as obtained by polymerizing a hydrophilic monomer; thereafter pulverizing the resultant dry polymer with a pulverizer, such as a roll mill, a high-speed-rotary pulverizer, or a jet pulverizer; and further, classifying and transferring the resultant pulverized product. In these steps, the dried product of the crosslinked hydrogel polymer hurts metallic faces to generate the metallic foreign substance.
When the water-absorbent resin powder is used for sanitary materials, the water-absorbent resin powder having a particularly narrow particle diameter distribution and a low fine-powder content is favorable because it has excellent properties. Accordingly, the dried product of the crosslinked hydrogel polymer is pulverized, and thereafter its particle diameter distribution is arranged to the above favorable one in a sieving classification step. At this stage, if an attempt is made to decrease the fine-powder content by raising classification accuracy in the sieving classification step in order to enhance an SFC (a saline flow conductivity) of the water-absorbent resin, then the sieve is easily hurt. Such a tendency is more remarkable in the case where at least two sieves are used, particularly, in at least two places.
The above deterioration is particularly easily caused when the load to the production line of the water-absorbent resin powder is so large as to continuously produce the water-absorbent resin powder at a productivity of not less than 500 kg/hr per line.
As to the above deterioration, the metallic foreign substance is particularly easily generated when an attempt is made to obtain powder particles of the irregular shape having a specific narrow particle diameter distribution. The crosslinked hydrogel polymer is preliminarily divided into small pieces before pulverization. However, if the crosslinked hydrogel polymer obtained by carrying out shearing polymerization and/or static polymerization is divided into small pieces, the resultant dry product is of the irregular shape or agglomerated, and is therefore so angular as to easily hurt particularly the metallic faces of the apparatus.
The water-absorbent resin is generally surface-crosslinked to thereby improve its water absorption performance. However, in this surface-crosslinking step, particularly, if this surface-crosslinking is carried out at 150 to 250xc2x0 C., the hardness of the water-absorbent resin powder is increased because the solid content of the water-absorbent resin is raised. Therefore, particularly, the contamination with the metallic foreign substance is easily caused because the resultant water-absorbent resin powder still more easily hurts the metallic faces of the apparatus.
The contamination with the metallic foreign substance may be caused also when the crosslinked hydrogel polymer powder is transferred or stored during or after the production of this powder.
The present inventors proposed the use of a sieving device in a heated and/or heat-retained state in order to classify a dried product of a crosslinked hydrogel polymer as to particle size with high accuracy and good productivity (JP-A-202187/1998 and EP 855232). When the sieving classification treatment of the dried product of the crosslinked hydrogel polymer is carried out at a high temperature, the resultant dried product comes to have such a low water content as to be hard. Therefore, the contamination with the metallic foreign substance is easily caused in a large amount.
A production process for a water-absorbent resin powder according to the present invention, which has been completed after having come through such as the above consideration and experimental confirmation, comprises the steps of: polymerizing an unsaturated monomer; and drying the resultant crosslinked hydrogel polymer, wherein the water-absorbent resin powder has a crosslinked structure and a mass-average particle diameter of 300 to 600 xcexcm wherein the ratio of fine particles having particle diameters of not larger than 150 xcexcm is less than 10 mass % of the water-absorbent resin powder, with the production process being characterized by further comprising a magnetic-flux irradiation step of passing a magnetic field having a magnetic flux density of not less than 0.05 Wb/m2 through the water-absorbent resin powder after the drying step in the production process.
The present invention production process enables easy production of the above present invention water-absorbent resin powder. Particularly, if the water-absorbent resin powder is exposed to the magnetic flux in the present invention production process, then there can be removed the metallic foreign substance, especially, fine particles of the metallic foreign substance having particle diameters of not larger than 1 mm, and further especially, even a fine iron powder and a stainless metallic foreign substance that cannot be seen with the eye.
As to the present invention production process for a water-absorbent resin powder, it is favorable that this process, after the drying step, further comprises a pulverization step, a classification step, and continuous transferring steps of connecting the steps of the production process to each other, wherein the magnetic-flux irradiation step is included after the classification step.
As to the present invention production process for a water-absorbent resin powder, it is favorable that the irradiation of the magnetic flux is carried out in at least two places on the continuous transferring steps, and that one of the at least two places is located just upstream of a hopper for storing an end product of the water-absorbent resin powder.
Furthermore, as to the present invention production process for a water-absorbent resin powder, it is favorable that the magnetic flux is a magnetic flux having a magnetic flux density of not less than 1.0 Wb/m2, and it is favorable that the irradiation of the magnetic flux is carried out with a permanent magnet and/or an electromagnet.
The present invention production process for a water-absorbent resin powder displays remarkable effects when the load to the production line is so large as to continuously produce the water-absorbent resin powder at a productivity of not less than 500 kg/hr per line.
A water-absorbent resin powder, according to the present invention, is a water-absorbent resin powder of which the major proportion is comprised of a polymer that is obtained by a process including the step of polymerizing a monomer including acrylic acid and/or its salt in a major proportion, and which has a mass-average particle diameter of 300 to 600 xcexcm wherein the ratio of fine particles having particle diameters of not larger than 150 xcexcm is less than 10 mass % of the water-absorbent resin powder, and which has a crosslinked structure and is irradiated with a magnetic flux.
A sanitary material, according to the present invention, comprises a water-absorbent resin powder of which the major proportion is comprised of a polymer that is obtained by a process including the step of polymerizing a monomer including acrylic acid and/or its salt in a major proportion, and which has a mass-average particle diameter of 300 to 600 xcexcm wherein the ratio of fine particles having particle diameters of not larger than 150 xcexcm is less than 10 mass % of the water-absorbent resin powder, and which has a crosslinked structure and is irradiated with a magnetic flux.